Novel directional coupler for high-frequency electric signals

ABSTRACT

An electromechanical directional coupler for high-frequency waves. Couplers of this kind comprise an input transducer and a number of output transducers disposed on a piezoelectric member in the path of the wave, and a number of conductive strips between the first transducer and the other transducers, so as to distribute among the output transducers the energy of the wave applied to the input transducer. According to the invention, the strips are obtained by conductivity induced by a beam of electrons which bombards the piezoelectric member or an electrically insulating member which covers member. The same device operates at various frequencies, the coupling ratios between the input transducer and the output transducers being adjustable. General application to high-frequency directional coupling.

United States Patent 1191 Bert et a]. [451 Oct. 28, 1975 NOVELDIRECTIONAL COUPLER FOR Primary Examiner-Paul L. Gensler HIGH-FREQUENCYELECTRIC SIGNALS A170" g fl PIOIICI [75] Inventors: Alain Bert; GerardKantorowicz, 1 both of Paris, France ABSTRACT [73] Assignee: ThomsomCSF,Paris France An electromechanical directional coupler for highfrequencywaves. [22] Flled' July 1974 Couplers of this kind comprise an inputtransducer [21] Appl. No.: 492,777 and a number of output transducersdisposed on a piezoelectric member in the path of the wave, and a numberof conductive strips between the first [3O] Forelgn Apphcamm Pnomy Datatransducer and the other transducers, so as to Aug. 2, France distributeamong the output transducers the energy of the wave applied to the inputtransducer. According 52 US. (:1. 333/10; 333/30 R to the invention, theships are obtained 1 by 51 Int. Cl. 1101P 5/04;H01P 5/18 Conductivityinduced by a beam. f electrons which [58] Field of Search 333/10, 30 Rbombards the piezoelectric member or an electrically insulating memberwhich covers member. The same [56] References C'ted device operates atvarious frequencies, the coupling UNITED STATES PATENTS ratios betweenthe input transducer and the output 3,516,027 6 1970 Wasilik .1. 333/30R transducers being adjustable- 3,560,891 2/1971 MacLeay at al. 333/10 XGeneral application to high-frequency directional 3,585,531 6/l97lDegenford et al. 333/10 X coupling 1 3,836,876 9/1974 Marshall et al.333/ R 8 Claims, 5 Drawing Figures ELECTRON 37 BEAM ASSEMBLY .10 3

070 6 IIPIEZOELECTRIC 0] 077 04 v 020 i 02 7 027 Q 1 030 I 1 I I I 2903/ ELECTRICALLY 77177 INSULATING MATERIAL U.S. Patent Oct. 28, 1975Sheet 1 of2 3,916,347

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PRIOR ART /0 5 50 I L m 1 1 ,020 \J' IEVOZ 071 f L02] M030 0&1 [E 03 oslLE TRON BEAM ASSEMBLYIQO 330 32 010 5 (IKEIEZOELECTRIC o1 on 04 020 030I) /I/ l 031" 25 60 34 ELECTRICALLY INSULATING MATERIAL U.S. PatentOct.28, 1975 Sheet2of2 3,916,347

NOVEL DIRECTIONAL COUPLER FOR HIGH-FREQUENCY ELECTRIC SIGNALS Theinvention relates to a directional coupler comprising a piezoelectriccrystal for high-frequency waves.

There are known delay devices comprising a piezoelectric crystal forhigh-frequency waves. The known devices comprise a piezoelectric memberon which input and output transducers are applied. When an electricsignal is applied to the input transducer, it induces a mechanical wavein the piezoelectric member, the wave having the same frequency as theapplied signal and propagating at the surface of the member. The wave,which is accompanied by a potential wave, reaches the output transducerand in turn induces a signal which is collected at the terminals of theoutput transducer. The last-mentioned signal is somewhat delayedcompared with the input signal owing to the time taken by the mechanicalwave to propagate at the surface of the piezoelectric member. Referenceshould be made to the technical literature for details about this kindof devices, which are also called electromechanical or electro-acoustic.

In the prior art, devices of the aforementioned kind are used fordirectional coupling between an input transducer and one or more outputtransducers. See Novel Acoustic Surface Wave Directional Coupler withDiverse Applications, Electronics Letters, 12 Aug. 71, Vol. 7, No. 16,pages 460/462, F. G..Marshall, E. G. Paige, and Surface Acoustic WaveMultistrip Components and their Applications, IEEE Transactions onMicrowave Theory and Techniques, Vol. 21, No. 4, April 1973, F. G.Marshall et al.

In the prior art, delay electroacoustic devices are used to providedirectional coupling of high-frequency waves by inserting a number ofconductive strips between the input transducer and the outputtransducers on the surface of the piezoelectric member, the strips beinglarge compared with the transducers in a direction at an angle, e.g.perpendicular, to the direction of propagation of the surface wave inquestion.

In the prior-art devices, however, the conductive strips are in the formof conductors which are applied in stationary manner to thepiezoelectric member. The conductors are conventionally obtained bydepositing metal on the surface of the piezoelectric member.

Since the deposits are stationary, the resulting directional couplingcharacteristics are also stationary.

It may be advantageous, using the same delay electromechanical device,to obtain directional couplings having characteristics which varydepending on the use made of the device.

"An object of the invention is to describe a directional couplercomprising a piezoelectric crystal and having adjustablecharacteristics.

The following description refers to the accompanying drawings whichdiagrammatically show in:

FIG. 1, a plane view of a device of the prior art;

FIG. 2, a perspective view of a device of the invention; and

allel conductive strips 5 forming an assembly 50 are disposed betweenthe transducers and also applied to the surface of member 1. Thetransducers and strips are obtained e.g. by depositing metal on thesurface of member 1 and occupy stationary positions thereon.

A high-frequency signal having a given frequency is applied betweenterminals 010 and 011 of transducer 1 and produces a surface mechanicalwave which propagates along the piezoelectric member. If there were nostrips 50, the wave would propagate on the surface of the piezoelectricmember in an invariable direction depending on the orientation of themember. In the drawing, this direction is represented by arrow 10.

The wave would then induce a signal corresponding to the input signal intransducer 02.

Owing to the presence of strips 5 which, as shown in the drawing, arelarge compared with the transducers in the direction perpendicular 'tothe propagation direction, the wave is deflected downwards.

The assembly 50 of strips 5 behaves like two guides for waves which areguided in the direction of the arrow, the guides being adjacent andcoupled together by apertures which are formed in their common surfaceand which can occupy either some or all the area of this surface. Thetwo guides are e.g. two identical rectangular wave guides whose shortsides are adjacent and parallel to the direction 10 in the central planeof the FIGS. 3, 4 and 5, partial plane views of three embodiments of theinvention.

FIG. 1 is a diagrammatic plan view showing a piezoelectric member I,e.g. a known quartz wafer cut in a privileged direction, and fourtransducers 01,02, 03, 04 applied to member 1. A number of equidistantpardrawing.

It is known that, in this case, the wave initially propagates along thedirection of arrow 11 and is gradually attenuated; owing to theaforementioned coupling, a wave at the same frequency appears in thesecond guide along arrow 12; the latter wave induces a signal intransducer 03. The result is a coupling between transducer 01 andtransducer 03, which adversely affects the coupling in relation totransducer 02 which would occur in the absence of strips 50. However,the attenuation along the path 11 varies with the length of the waveguide sections; if the wave guide sections are short it may be partial,whereas if the sections are sufficiently long it may affect the entireinitial wave. Similarly, returning to the diagram in FIG. 1, we see thatthe attenuation along path 11 andthe resulting directional effectdepends on the number of strips 5. Incidentally, if the guide sectionsare even longer, i.e. if there is a sufficient number of strips 5, mostof the energy returns along path 11, in which case we shall again have apreferential coupling between transducers 01 and 02.

Consequently, the directionalcharacter of the coupling obtained by theprior-art devices depends on the number of strips 5 forming assembly 50;the proportion of energy received from transducer 01 by transducers 02and 03 is dependent on the number of strips 5. The strips, which arenarrow, are usually disposed at a distance of M3 from one another, Abeing the wavelength of the aforementioned signal.

When the strips are metal conductors applied to the surface of member 1,as in the prior art shown in FIG. 1, the coupling ratio of transducer 01with each of transducers 02 and 03 is stationary for a given device.This is a disadvantage. Similarly, the operating frequency associatedwith the spacing between the strips is substantially stationary. This isanother disadvantage.

The electroacoustic device for unidirectional coupling according to theinvention is designed to obviate these disadvantages. This object isobtained in a manner which will be described hereinafter, with referenceto FIG. 2.

FIG. 2 is a perspective diagram, showing a wafer 1 of piezoelectricmaterial on which four transducers are disposed as in FIG. 1 and bearthe same references as in FIG. 1.

A number of strips 6 represented by dotted rectangles are disposedbetween the transducers and form an assembly 60. The strips serve thesame purpose as the strips in the preceding exemple. They aremanufactured as shown hereinafter.

A layer of a material having the property of induced conductiveness whenbombarded by electrons is disposed on the piezoelectric material 1. Theproperty referred to belongs to certain materials, which, when bombardedby an electron beam, become conductive at the place of impact of thebeam and for a certain distance below the place of impact. Suchmaterials are known in electronics; an example is cadmium sulphide orCdS, which is used to make layer 20 in the example given.

Means are provided for producing a bombardment of the aforementionedkind in the device according to the invention. The means comprise athermionic cathode 31 having a heating filament (not shown), a controlelectrode 32, an electrode 33 and an electrode 34. The beam from cathode31 is accelerated towards layer 20 by a potential difference producedbetween electrodes 31 and 34 as shown in the drawing, by using a source(not shown) whose negative terminal is connected to the electrode 31 andwhose positive terminal is connected to anode 34, i.e. to earth in theexample shown. Electrode 33 is in two parts and is raised to a potentialwhich is between that of cathode 31 and earth, and can thus deflect thebeam by producing a potential difference between its two parts, by meansof connections 330 to a source (not shown), whereas electrode 32 is usedfor periodically interrupting the beam, as will be seen hereinafter. Thepotential of the layer is fixed by a collector which collects thesecondary electrons emitted by layer 20 when bombarded. For the sake ofsimplicity, the drawings do not show the collector, which may either bea grid parallel to the layer 20 through which the'incident electronstravel, or a conductive deposit on the periphery of layer 20, or may beembodied in any other prior-art manner.

The gun assembly 30 may also comprise other electrodes, e.g. collimationelectrodes, in accordance with any known electron-gun feature. FIG. 2does not show any of these electrodes, since they are not necessary forunderstanding the operation of the device. FIG. 2 likewise does not showthe negative-pressure casing inside which the bombardment occurs.

When assembly 30 is energized and electrode 32 is at a suitable voltage,an electron beam travels from cathode 31 to layer 20. The beam, which isin the form of a sheet, is limited at the front and rear of the drawingby curved lines (not referenced). It produces an impact on layer 20,represented by one of the dotted rectangles 6.

The electron beam bombards layer 20 at an energy of several kilovolts,e.g. 4 kilovolts in the case of cadmium suphide in the example. Owing tothe bombardment, the conductivity of layer 20 increases at the place ofimpact of the electron beam, since free charge carriers are produced inthe mass of layer 20 under the surface where the electrons impinge, thenumber of carriers depending on the nature of the layer material. In thecase of cadmium sulphide (CdS), the number of carriers at theaforementioned acceleration voltage is about 1,000 times the number ofincident electrons. The free carriers are distributed in the material toa depth not exceeding one tenth of a micron.

In the case of a beam having an intensity of e.g. 1 microampere, a pulselasting 10 microseconds and an impact cross-section of approx. 10 mm X0.03 mm (the size of rectangles 6), the number of free carriers producedper pulse is about 2 X 10 per cubic centimetre, corresponding to aresistivity of the order of 0.1 ohm cm.

The electron beam coming from cathode 31 is chopped by grid 32 intopulses each lasting 10 microseconds and repeating every thousandth of asecond. The two plates forming electrode 33 scan at the mains frequency,i.e. 50 cycles per second, the beam making an outward and returnmovement, per cycle of one hundredth of a second each. During thisduration, there are 10 pulses from the control grid, each correspondingto a strip 6; accordingly 10 strips 6 are produced on layer 20 during ascanning cycle. For clarity, only some of the strips have been shown.The conductivity of the strips is renewed at each transit. of theelectron beam. The strips remain permanently conductive during scanning.provided that the recombination time of the free carriers produced inlayer 20 during the transit of the electron beam through a strip issubstantially greater than the time between two successive transits ofthe beam through the strip. Accordingly, the beam travels a second timethrough the strip before the conductivity produced therein by theprevious transit has had time to disappear owing to the recombination ofthe free carriers. To obtain satisfactory operation, this time shouldalso be substantially greater than the period of the acoustic wave. Thefirst of these conditions can easily be obtained from the aforementioneddata and results in the second condition at the operating frequencies,e.g. 50 MHz.

The conductivity, however, disappears when the strip cease to be scannedfor a time longer than the recombination time; according to thepreceding data, the recombination time is quite short, i.e. a fewthousandths of a second. Consequently, if a number of strips 6 areformed on layer 20, they can be erased if they are no longer maintainedby the electron beam. Consequently, after a first set of strips 6 hasbeen produced, a different set can be produced without the devicepreserving any trace of the preceding set.

In other respects, the device in FIG. 2 operates in the same manner asthe device in FIG. 1. In the device in FIG. 1, however, the strips 5forming assembly 50 and represented by conductors deposited on thesurface of substrat l occupy a stationary position thereupon, whereas inthe device in FIG. 2 the strips 6 which serve the same purpose as strips5 in FIG. 1 can be moved on film 20 by acting in known manner on thefrequency of the current pulses and on the scanning characteristics ofthe electron beam and film 20. It is thus possible to displace thestrips with respect to one another, i.e. to modify the pitch of thearray formed by assembly 60 of strips 6 and to vary the number ofstrips, etc. Consequently, couplers according to the invention havedifferent properties from similar prior-art couplers.

More particularly, if the spacing between strips 6 is modified, a singlecoupler can operate at different freresults in a modification in thecoupling ratio between the input transducer and the two outputtransducers 02 and03, i.e; in the proportion of the wave energy 'appliedto the input transducer which is collected by each of the outputtransducers. As already stated, the number of strips can be-varied so asto transfer practically .all the initial wave from pathv 11 to path 12,i.e. towards terminals 030 and 031 of transducer'03, or alternatively totransfer only'part of the initial wave to transv ducer 03, the remaindergoingto transducer 02 and being sampled between the terminals 020 and021 thereof. I

Similarly, as in the example diagrammatically illustrated in plan'viewin FIG. 3, strips 6 can beprovided which are partly in the form ofconductors 61 on the surface of material 1 and partly in the form ofregions 62 of a layer (not shown) made conductive by inducedconductivity; each region 62 is disposed between two portions ofconductors 61, as shown in the drawing. Coupling occurs only when theelectron beam bombards regions 62; no coupling occurs in the absence ofsuch bombardment. In this case, the regions need not necessarily bescanned and there can be a permanent bombardment during the whole timewhen the filter is used.

In the example in FIG. 4, the piezoelectric material has the property ofconductivity induced by electron bombardment, so that layer 20 isunnecessary.

FIGS. 4 and 5 show two other embodiments of cow plers according to theinvention.

In FIGS. 4 and 5, the piezoelectric member 1 comprises two adjacentparts 100 and 101 which are normally insulated from one another withregard to the transmission of the electromechanical wave. Each part 100and 101 bears metal conductors 61 as in the preceding example, theconductors being diagrammatically shown as fixed lines. Usually, eachpart operates separately and provides coupling (which is not in any waydirectional) between the transducers at the two ends thereof. Whenelectric conduction between a conductor 61 on one part and a conductoron the facing part is produced by bombarding the regions 62 shown bydotted circular surfaces in the drawing, directional coupling occursunder the conditions explained with reference to FIGS. 1 and 2. As inthe example in FIG. 3, the electronic bombardment can be maintainedduring the whole time during which directional coupling is required.

FIG. 5 is an alternative embodiment of FIG. 4, wherein the two adjacentparts 100 and 101 are made of different piezoelectric materials havingdifferent propagation rates. The two parts are coupled in the same wayas before. Note that the last-mentioned feature would be difficult toobtain using an electron beam bombardment to produce the strips 6 intheir whole by induced conductivity.

We have now described a number of embodiments of the invention. Theinvention also includes all other embodiments which can be devised bythe skilled addressee. v

The devices according to the invention comprise a piezoelectric memberwhich, at the system of strips 60, is covered with a material such ascadmium sulphide,

. which can be given induced conductivity. Thesame devices can beconstructed as mentioned in connection with, FIG. 4,- using a singlematerial which is both piezoelectric andcan be given induced conductivity.Cadmium sulphideitself can be used. for this purpose, and galliumarsenide is equallyusefuh .In the examples described, we haveassumedthat there are two outputtransducers. Allthe foregoing appliesequally well to devices comprising more than two output transducers.

Of course, .the invention is not limited to the embodiment described andshown which was given solely by ----way of example.

What is claimed is: r 1. A-directional coupler for high-frequencyn-wavescomprising, on a piezoelectric member, an input transducer to'which ahigh-frequency wave is applied and a number of output transducerscollecting thecorresponding wave transmitted to the surface of thepiezoelectric-member, and a number of conductive strips disposed betweenthe input transducer and the output transducers at an angle to thedirection of the highfrequency wave in the piezoelectric member,characterised in that it comprises means producing an electron beam,means causing the beam to strike regions of said piezoelectric member,means to endow said regions with the property of being electricallyinsulating, said regions being made conductive at the place of impactowing to free charge carriers being produced therein, means for choppingthe beam into pulses, the impact of the beam on each region occuringduring one of the pulses, and means causing the beam to periodicallyscan the regions at'a period less than the recombination time of thefree'charge carriers.

2. A directional coupler according to claim 1, characterised in thatsaid means to endow said regions with the property of being electricallyinsulating is a layer of an electrically insulating material coveringsaid piezoelectric member on that of its faces struck by the electronbeam.

3. A directional coupler according to claim 1, characterised in thateach conductive strip is made up of two portions of metal conductorsseparated from one another and a region which is made conductive byelectron bombardment and is in contact with the aforementioned twoportions.

4. A directional coupler according to claim 3, characterised in that thepiezoelectric member is made up of two separate pieces placed side byside and connected transversely to the aforementioned portions, each ofthe two portions forming one of the strips being at least partlydisposed on one of the aforementioned two pieces.

5. A directional coupler according to claim 4, characterised in that thetwo pieces making up the piezoelectric member are made of the samematerial.

6. A directional coupler according to claim 4, characterised in that thetwo pieces making up the piezoelectric member are made of differentmaterials.

7. A directional coupler for high-frequency waves comprising, on apiezoelectric member, an input transducer to which a high-frequency waveis applied and a number ,of output transducers collecting thecorresponding wave transmitted to the surface of the piezoelectricmember, and a number of conductive strips disposed between the inputtransducer and the output transducers at an angle to the direction ofthe highfrequency wave in the piezoelectric member, characterised inthat it comprises means producing an electron beam, means causing thebeam to strike regions of said piezoelectric member, means to endow saidregions with the property of being electrically insulating, said regionsbeing made conductive at the place of impact owing to free chargecarriers being produced therein, means for chopping the beam intopulses, the impact of the beam on each region occuring during one of thepulses, and means causing the beam to periodically scan the regions at aperiod less than the recombination time of the free charge carriers, thedevice also comprising means associated with the deflecting means formodifying the spacing between the regions and thus enabling the couplerto be used for high-frequency waves having varying frequencies.

8. A directional coupler for high-frequency waves comprising, on apiezoelectric member, an input transducer to which a high-frequency waveis applied and a number of output transducers collecting thecorresponding wave transmitted to the surface of the piezoelectricmember, and a number of conductive strips disposed between the inputtransducer and the output transducers at an angle to the direction ofthe highfrequency wave in the piezoelectric member, characterised inthat it comprises means producing an electron beam, means causing thebeam to strike regions of said piezoelectric member, means to endow saidregions with the property of being electrically insulating, said regionsbeing made conductive at the place of impact owing to free chargecarriers being produced therein, means for chopping the beam intopulses, the impact of the beam on each region occuring during one of thepulses, and means causing the beam to periodically scan the regions at aperiod less than the recombi' nation time of the free charge carriers,the device also comprising means for modifying the number of theaforementioned regions and consequently modifying the coupling ratio ofeach output transducer with the input transducer.

1. A directional coupler for high-frequency waves comprising, on apiezoelectric member, an input transducer to which a highfrequency waveis applied and a number of output transducers collecting thecorresponding wave transmitted to the surface of the piezoelectricmember, and a number of conductive strips disposed between the inputtransducer and the output transducers at an angle to the direction ofthe high-frequency wave in the piezoelectric member, characterised inthat it comprises means producing an electron beam, means causing thebeam to strike regions of said piezoelectric member, means to endow saidregions with the property of being electrically insulating, said regionsbeing made conductive at the place of impact owing to free chargecarriers being produced therein, means for chopping the beam intopulses, tHe impact of the beam on each region occuring during one of thepulses, and means causing the beam to periodically scan the regions at aperiod less than the recombination time of the free charge carriers. 2.A directional coupler according to claim 1, characterised in that saidmeans to endow said regions with the property of being electricallyinsulating is a layer of an electrically insulating material coveringsaid piezoelectric member on that of its faces struck by the electronbeam.
 3. A directional coupler according to claim 1, characterised inthat each conductive strip is made up of two portions of metalconductors separated from one another and a region which is madeconductive by electron bombardment and is in contact with theaforementioned two portions.
 4. A directional coupler according to claim3, characterised in that the piezoelectric member is made up of twoseparate pieces placed side by side and connected transversely to theaforementioned portions, each of the two portions forming one of thestrips being at least partly disposed on one of the aforementioned twopieces.
 5. A directional coupler according to claim 4, characterised inthat the two pieces making up the piezoelectric member are made of thesame material.
 6. A directional coupler according to claim 4,characterised in that the two pieces making up the piezoelectric memberare made of different materials.
 7. A directional coupler forhigh-frequency waves comprising, on a piezoelectric member, an inputtransducer to which a high-frequency wave is applied and a number ofoutput transducers collecting the corresponding wave transmitted to thesurface of the piezoelectric member, and a number of conductive stripsdisposed between the input transducer and the output transducers at anangle to the direction of the high-frequency wave in the piezoelectricmember, characterised in that it comprises means producing an electronbeam, means causing the beam to strike regions of said piezoelectricmember, means to endow said regions with the property of beingelectrically insulating, said regions being made conductive at the placeof impact owing to free charge carriers being produced therein, meansfor chopping the beam into pulses, the impact of the beam on each regionoccuring during one of the pulses, and means causing the beam toperiodically scan the regions at a period less than the recombinationtime of the free charge carriers, the device also comprising meansassociated with the deflecting means for modifying the spacing betweenthe regions and thus enabling the coupler to be used for high-frequencywaves having varying frequencies.
 8. A directional coupler forhigh-frequency waves comprising, on a piezoelectric member, an inputtransducer to which a high-frequency wave is applied and a number ofoutput transducers collecting the corresponding wave transmitted to thesurface of the piezoelectric member, and a number of conductive stripsdisposed between the input transducer and the output transducers at anangle to the direction of the high-frequency wave in the piezoelectricmember, characterised in that it comprises means producing an electronbeam, means causing the beam to strike regions of said piezoelectricmember, means to endow said regions with the property of beingelectrically insulating, said regions being made conductive at the placeof impact owing to free charge carriers being produced therein, meansfor chopping the beam into pulses, the impact of the beam on each regionoccuring during one of the pulses, and means causing the beam toperiodically scan the regions at a period less than the recombinationtime of the free charge carriers, the device also comprising means formodifying the number of the aforementioned regions and consequentlymodifying the coupling ratio of each output transducer with the inputtransducer.